1. Field of the Invention
The present invention relates to an LCD panel, and in particular to an LCD having a common voltage adjustment.
2. Description of the Related Art
FIG. 1A is an equivalent circuit diagram of a pixel 10 of the conventional TFT-LCD. In FIG. 1A, the circuit of each pixel has a data line (DL), bit line (BL), thin film transistor (M), compensation capacitor (C) and equivalent capacitor (Lc). The data line (DL) is connected to the source terminal (S) of the thin film transistor (M). The bit line (BL) is connected to the gate terminal (G). The equivalent capacitor (Lc) produced by the liquid crystal in the LCD panel and the compensation capacitor (C) are disposed between the drain terminal (D) and the ground terminal.
FIG. 1B is an output waveform of the conventional TFT-LCD. VDL represents the voltage of the data line (DL). VBL represents the voltage of the bit line (BL). Vc is the average voltage of the data line (DL). Vcom is the voltage of the ground terminal. Vo is the voltage over the equivalent capacitor (Lc). Moreover, T1 is the selected period, and T2 is the non-selected period of the bit line (BL).
Referring to FIGS. 1A and 1B, during the selected period (T1), VBL is high, and the gate terminal (G) of the thin film transistor (M) is in forward-active mode, such that the voltage (Vo) over the equivalent capacitor (Lc) is equal to the voltage (VDL) of the data line. At the time when VBL becomes low, the gate terminal (G) is cutoff. The voltage (Vo) drops by ΔV immediately because of the parasitic capacitor between the gate terminal (G) and the drain terminal (D) of the thin film transistor (M). Further, the voltage Vo decays within the non-selected period (T2) because of the current leakage. After VBL becomes high again, the equivalent capacitor (Lc) are re-charged.
Because the voltage drop ΔV has no relationship with the direction of the data line voltage (VDL), the output voltage (Vo) always drops a voltage by ΔV, which depends on the fabricating conditions of the thin film transistor (M). Therefore, the output voltage (Vo) is not symmetrical. The TFT-LCD panel may flash, and the illumination of the image is unstable.